Process for forming piles and product

ABSTRACT

A method of forming a pile comprises driving a pile forming member (12, 40, 140) into the ground to a predetermined depth to form a passage of non-circular cross-section, preferably a circular cross-section (42) with fins (56) radiating therefrom, and while withdrawing the member (12, 40, 140) from the passage filling the passage with cementitious material so that then this material sets a pile if formed in the passage.

The present invention relates to piles and to their production.

Piles for use in supporting structures such as buildings are commonlypre-formed, in sections, and driven into the ground from the surface.Alternatively, they may be formed by driving a head into the earth toform a hole. A pipe extending down the hole supplies concrete or otherpile material to the head. The head comprises a through passage whichallows the pile material to fill the hole beneath the head as the headis withdrawn from the hole. A sacrificial cap closes the lower end ofthe through passage while the head is being driven to form the hole, andis left at the bottom of the hole when the head is withdrawn.

A major contribution to the supporting force provided by a pile arisesfrom friction between the surface of the pile and the surrounding earth.This friction can be increased by increasing the size of the pile, butthere is a resultant increase in the volume of pile material required.It is an object of the present invention to obviate or mitigate thisproblem.

According to the present invention there is provided a method of forminga pile comprising applying a downwardly directed force to a pile formingmember to drive said member into the ground top a predetermined depthcharacterized in that said pile forming member has three or four finsprojecting transversely thereof and is so driven into the grounds thatthe passage it forms in the ground has a cross-section corresponding tothat of the member and fins and in that an upwardly directed vibratoryforce is applied to withdraw said member from the ground whilesimultaneously supplying a pile forming material through the member tofill said passage as the member is removed therefrom thereby creating apile having a cross-section corresponding to that of the member andfins.

Preferably the pile member is driven into the ground by applying adownwardly directed vibratory force thereto.

Preferably a reinforcing member is positioned in the passage before thepile forming material sets.

Preferably the cross-section of the member is triangular.

Each fin is superimposed on said triangular cross-section member.Alternatively each fin is superimposed on an outline which is otherwisecircular.

Further according to the present invention there is provided a pileformed in accordance with the method set out in the proceeding fiveparagraphs.

Embodiments of devices according to the present invention, and the pilesproduced by those devices will now be described in more detail, by wayof example only, and with reference to the accompanying drawings inwhich:

FIG. 1 is perspective view of a pile-making head according to theinvention;

FIG. 2 is a cross-section of a pile formed using the head of FIG. 1; and

FIG. 3 is an elevation of a further alternative head.

FIG. 1 shows a pile-making device 10 comprising a head 12 which, in use,is driven into the earth to form a hole. Means 14 in the form of a pipeare provided for supplying pile material (normally concrete) to the head12. A through passage 16 within the head allows the pile material tofill the hole beneath the head 12 as the head is withdrawn from thehole. The device 10 may be driven into and out of a hole by anyconventional technique, such as hammering, jacking or vibrating but avibrating technique is preferred certainly for withdrawing the devicefrom the hole.

The outermost outline of the head 12 is so shaped as to form a hole ofnon-circular cross-section.

In more detail, the head 12 comprises a block 18 in the form of atriangular prism whose central axis forms the axis of the head and theaxis of the hole formed by the head. One end of the block 18 carries anextension 20 in the approximate form of a pyramid which causes the headto tapper from the outline of the block 18 to the mouth of the throughpassage 16, shown closed by a sacrificial cap 22.

The base of the pyramid is slightly larger than the cross-section of theprism, so that a small step 23 is formed where they meet. The outline ofhead is at its largest at this point. That is, the outermost outline ofthe head, when viewed along the axis of the prism, occurs only at onepoint along the length of the head, at the top of the step 23.

Three fins 24 project radially from the axis of the head 12 and aregenerally aligned in the radial planes of the edges of the block 18. Itcan be seen from FIG. 1 that the fins 24 project beyond the outermostoutline of the block 18 and that they extend substantially further inthe radial direction than in the circumferential direction. The fins maybe formed, for instance, by sections of steel plate.

The head 12 is used to form a pile in the following manner. The head 12is driven into the earth by a vibrator to form a hole to the requireddepth. The extension 20 and cap 22 lead as the head 12 is being driven.The pipe 14 follows the head 12 down the hole, additional sections ofpipe being added as required. Since the outermost outline of the head12, in crosssection, is triangular, with the exception of the fins 24,the head forms a generally triangular hole 26 (FIG. 2) which is extendedat each corner by slots 28 formed by the fins 24. When a hole 26 to thedesired depth has been formed, the head is withdrawn by reversing thedirection of force applied by the vibrator as concrete is suppliedthrough the pipe 14 and the through passage 16 to the space left in thehole below the head 12. This space is filled with concrete to leave apile 30 which is generally triangular in cross-section, with additionalflanges 32 running in each slot 28.

The taper of the pyramid 20 and the step 23 minimise the frictionresistance while driving the head 12 to form the hole 26. It may benecessary to provide some means for preventing the hole collapsingbehind (above) the head 12, around the pipe 14. These means could beplates which form an outline slightly smaller than the outline of theblock 18, or the triangular prism 12 could be extended in sections backto the surface.

The frictional force between the pile 30 and the surrounding earth isproportional to the surface area of the pile 30. It will be seen fromthe following explanation that the surface area of a specified volume ofconcrete is greater when that volume is cast in the form shown in FIG. 2than when it is cast in the form of a conventional pile of circularcross-section. Consequently, the frictional forces and the load-bearingcapacity of the pile shown in FIG. 2 are greater than those of aconventional pile formed with the same volume of concrete.

The circumference of the cross-section of a circular pile of radius Rhas a length (2πR). The cross-sectional area is (πR²). The frictionbetween the pile and the earth will be proportional to the surface areaof the pile, and will therefore be proportional to (2πR) per unit lengthof the pile. The volume of concrete in the pile is (πR²) per unitlength.

The circumference of the cross-section of a triangular pile is 3S, ifthe length of each side of the triangle is S. The cross-sectional areais (√3/4S²). The friction created is therefore proportional to 3S perunit length, and the volume of concrete used is (√3/4S²) per unitlength.

If the volume of concrete per unit length is the same in each case, then##EQU1##

The following equation therefore relates the friction produced by thepiles: ##EQU2##

The friction produced by the same volume of concrete is therefore 28%higher when the concrete forms a triangular pile than when the concreteforms a circular pile. In addition, the pile shown in FIG. 2 has theflanges 32 which provide a large surface area, and therefore a largeincrease in friction, with only a relatively small increase in thevolume of concrete required.

A similar analysis to the one given above shows that a square pile(without flanges) has a performance enhanced by approximately 12%. Theperformance of a hexagonal pile (without flanges) in enhanced by about5%.

It is due to the increased surface area and consequent frictionalresistance created by the fins that the use of a vibrator to drive andwithdraw the pile is desirable. The pile cannot be simply pulled fromthe hole it has formed by applying an upwardly directed pull as theforce required would be too great for normal apparatus to produce. Avibrator can be reversed to readily provide sufficiently great upwardlydirected force.

Variations and modifications to the heads described above, andconsequently to the resulting piles, can be made without departing fromthe scope of the present invention.

A most useful number of fins is three and a head 140 with three fins 144is shown in FIG. 3. The fins 144 are substantially triangular in shapeand parallel sided. The leading edge 145 of each fin is substantiallyperpendicular to the axis of the pipe 146. Advantageously the pipe is ofthe form described in our co-pending U. K. Patent Application 8914764.

In each of the embodiments described above at least one steelreinforcing bar is located in the pile before the cementitious mixturehas set to provide reinforcement. In view of the relatively smalldiameter of the circular cross-section pile core this is an importantfeature.

I claim:
 1. A method of forming a pile, comprising:(a) providing a pileforming member having no less than three and no more than four finsprojecting transversely thereof; (b) applying a downwardly directedforce to said pile forming member to drive said member into the groundto a predetermined depth; (c) driving the pile forming member into theground to form a passage in the ground having a cross-sectioncorresponding to that of the member and fins; and (d) applying anupwardly directed vibratory force to withdraw said member from theground; (e) supplying a pile forming material through the membersimultaneously with the step of (d) to fill said passage as the memberis withdrawn therefrom thereby creating a pile having a cross-sectioncorresponding to that of the member and fins.
 2. A method of forming apile as claimed in claim 1, wherein the step of applying a downwardlydirected force to the member includes the step of applying a downwardlydirected vibratory force thereto.
 3. A method of forming a pile asclaimed in claim 1 or 2, and including the step of positioning areinforcing member in the passage before the pile forming material sets.4. A method according to claim 1, wherein the step of providing a pileforming member includes forming the pile forming member with atriangular cross-section.
 5. A method according to claim 4, wherein thestep of providing a pile forming member includes superimposing the finson said triangular cross-section member.
 6. A method according to claim1, characterized in that each fin is superimposed on an outline which isotherwise circular.
 7. A pile formed in accordance with the method ofclaim 1, 2, 4, 5, 6.